system_region_list() could use getTheme() instead of listInfo()

Is this something that will require a test case?

I'm not sure if there's a performance cost, but even if there isn't, it's clearer if the code just gets the one theme it wants.

There is no performance cost, because ThemeHandler::getTheme does the exact same thing. But I agree it would be clearer.

The latest patch is catching an InvalidArgumentException but that changed to \Drupal\Core\Extension\Exception\UnknownExtensionException in #2940203: Use dedicated Exception classes for extension system → . FWIW I'm not sure we should really be catching the exception, or if we are we should do something more useful than just returning an empty array. Otherwise you never really find out that something is broken.

Needs to be converted to an MR against 11.x.

Is this something that will require a test case?

Wouldn't have thought so, it should already be tested somewhere, we're just changing the implementation details.

Let's catch \Drupal\Core\Extension\Exception\UnknownExtensionException instead

Working on it.

use \Drupal\Core\Extension\Exception\UnknownExtensionException instead of \InvalidArgumentException.
please review.